Method and device for filter backwashing

ABSTRACT

A method of declogging at least one filter of a plant for manufacturing uranium oxide from uranium hexafluoride, including separating, from the wall of the filter, uranium oxyfluoride particles deposited, by a stream of inert gas such as nitrogen, injected into the filter, in a counter-currentwise direction to the flow of hydrofluoric acid.

[0001] The invention relates to a method and a device for declogging afilter of a plant for manufacturing uranium oxide from uraniumhexafluoride.

[0002] Uranium oxide manufacturing plants implementing a dry method ofmanufacturing uranium oxide from gaseous uranium hexafluoride UF₆ comingfrom a plant for enriching uranium used as a nuclear fuel withradioactive isotopes are known.

[0003] Such plants implementing the method of manufacturing uraniumoxide from uranium hexafluoride, called dry conversion method, comprise,in an industrial embodiment which is currently the most satisfactory andmost widespread embodiment, a reactor having an overall verticalarrangement into which uranium hexafluoride UF₆, steam and dilutionnitrogen are introduced, at a temperature of 200° C. to 300° C., anduranium hexafluoride UF₆ is converted into uranium oxyfluoride UO₂F₂according to the hydrolysis reaction:

UF₆+2H₂O→UO₂F₂+4HF.

[0004] The conversion of uranium hexafluoride into uranium oxyfluorideby hydrolysis produces hydrofluoric acid HF in gaseous form and uraniumoxyfluoride UO₂F₂ in powder form.

[0005] The uranium oxyfluoride UO₂F₂ falls and is deposited in thebottom of the reactor where it is taken up by a feed screw transferringit to the input of a rotary furnace in which the uranium oxyfluorideUO₂F₂ is converted into uranium oxides UO_(2+x), that is to say intouranium oxides UO₂ with a superstoichiometric composition, the O/U ratioof the oxide varying between 2.03 and 2.66, according to the conditionsfor carrying out the conversion in the rotary furnace.

[0006] Hydrofluoric acid HF is evacuated through the upper part of theuranium hexafluoride conversion reactor, through filtration units whichretain the uranium oxyfluoride powder UO₂F₂ conveyed by the gaseoushydrofluoric acid in a mixture with gases such as N₂ and H₂O suckedthrough the upper part of the conversion reactor.

[0007] Generally, at least two filtration units or blocks are used,arranged in the upper part of the reactor through which the gasescontaining hydrofluoric acid are evacuated.

[0008] Each of the filtration units comprises a gas evacuation manifold,or head, communicating via a pipe, outside the conversion reactor, witha plant enabling the gases to be routed toward a safety filter locatedoutside the reactor downstream of the gas routing plant, arranged in ahot box, and toward a unit for recovering hydrofluoric acid.

[0009] Inside the conversion reactor, the filtration units comprise aset of filters, or filter cartridges, each one comprising a filteringwall of overall cylindrical shape placed in the upper part of theconversion reactor with its axis vertical.

[0010] The filters, or filtering cartridges, of each of the filtrationunits rest, and are fastened, via an upper flange, onto a plate of thefiltration unit arranged horizontally and separating the manifold orhead of the filtration unit from the internal volume of the reactor inwhich the chemical reaction forming uranium oxyfluoride takes place.

[0011] The gases, loaded to a greater or lesser degree with powdereduranium oxyfluoride, come into contact with the outer surface of thefiltering cartridges and, after separation of the powdered uraniumoxyfluoride, cross the wall of the filtering cartridge thereby enteringthe head of the filtration unit. The powdered uranium oxyfluoridestopped by the outer wall of the filtration unit is able to fall back bygravity into the bottom of the reactor where it is taken up by the feedscrew for conveying the powder.

[0012] The tubular filtration walls of the filtering cartridges, whichgenerally consist of compressed and sintered particles or fibers, havepores to allow the passage of gases containing hydrofluoric acid, thesize of which is less than the size of the uranium oxyfluorideparticles, so as to stop all the solid particles which may be conveyedby the gases.

[0013] Some of the uranium oxyfluoride particles in powder form aredeposited on the outer surface of the tubular wall of the cartridge,such that the filters gradually clog up during plant operation. Passageof the mixed hydrofluoric acid and gases through the wall of the filtersoccurs with an increasing pressure drop which is incompatible withcontinuous operation of the plant.

[0014] It is therefore necessary to declog the walls of the filteringcartridges periodically and sufficiently frequently.

[0015] This declogging is carried out by injecting, inside the filteringcartridges, counter-currentwise with respect to the direction of flow ofthe gases containing the hydrofluoric acid, an inert declogging gas suchas nitrogen.

[0016] The declogging nitrogen must be delivered into the filteringcartridges with an overpressure of about 2.5 bar and at a temperature ofabout 130° C., the temperature in the conversion reactor being about300° C. and the temperature of the gases at the reactor outlet beingabout 200° C. to 300° C.

[0017] The declogging nitrogen which is contained in a storage tank isdelivered to the filtration units, via the gas routing plant locatedabove the conversion reactor. This gas routing plant is arranged insidea heated chamber, whose internal temperature is about 150° C.

[0018] Valves are arranged inside the heated chamber, in particularthree-way valves, making it possible, on the one hand, for the gasescontaining the hydrofluoric acid to pass to the safety filter and therecovery plant, during the phases of normal operation of the filtrationunit and, on the other hand, for the declogging nitrogen to pass in theopposite direction to the gas flow, during the declogging phases.

[0019] The valves withstand high temperatures and, because they operateat their upper temperature limit, their use and their maintenance arecomplex and expensive.

[0020] To allow continuous operation of the plant, declogging is carriedout on one of the filtration units while the second filtration unitalone evacuates the gases containing the hydrofluoric acid produced inthe reactor. As a result, during the declogging phases, the filtrationunit remaining in operation must evacuate a gas flow which is roughlytwice its usual operational flow. Furthermore, the flow of decloggingnitrogen, which is delivered into the second filtration unit, to theinside of the conversion reactor, must be evacuated, which furtherincreases the gas flow which must be evacuated by the filtration unitremaining in operation.

[0021] In one known embodiment of the filtration units of a uraniumhexafluoride conversion reactor, each of the filtration units compriseseight filtering cartridges which are distributed in the plane of thehorizontal plate for supporting the filters separating the head forrecovering hydrofluoric acid from the internal volume of the reactor.

[0022] Declogging nitrogen is injected into the head through the pipefor evacuating gases containing hydrofluoric acid, in a region locatedin the central part of the head, that is to say directly above thecentral part of the filter support plate. The result of this is that thejet of declogging nitrogen which is directed to the central part of theplate and to the filters carried by this central part carries outpreferential declogging of the filters located in the central part.These filters are perfectly declogged while the filters located on theperiphery of the plate supporting the filtration unit are notsufficiently declogged. These filters become fouled, so that thepressure increases and operation of the conversion plant is disturbed,the gases passing in a preferential manner through the filters locatedin the central part, which are efficiently declogged.

[0023] The aim of the invention is therefore to provide a method ofdeclogging at least one filter of a plant for manufacturing uraniumoxide from uranium hexafluoride, comprising a reactor in which gaseoushexafluoride UF₆ and steam are injected, uranium oxyfluoride UO₂F₂ inpowder form and gaseous hydrofluoric acid HF are formed, uraniumoxyfluoride UO₂F₂ conveyed by gases containing gaseous hydrofluoric acidHF is separated at the outlet of the reactor, in at least one filtrationunit arranged on an outlet part of the reactor, comprising at least onefilter having a tubular filtration wall arranged with its axis vertical,and hydrofluoric acid is evacuated to the outside of the reactor, thedeclogging of the filter consisting in separating, from the wall of thefilter, uranium oxyfluoride particles deposited, by a stream of inertgas such as nitrogen, injected into the filter, in a counter-currentwisedirection to the flow of hydrofluoric acid, this method making itpossible to carry out efficient declogging, by keeping in operation thefiltration unit on which is arranged the filter on which the decloggingis carried out, with simple means for routing gases and a small amountof inert declogging gas.

[0024] For this purpose, inert gas is injected along the axis of thewall of the filter in the form of a jet with a velocity greater than 300m/s, for a duration of less than 1 second.

[0025] For the invention to be better understood, a plant formanufacturing uranium oxide by the dry conversion method and adeclogging device making it possible to implement the method of theinvention will be described, by way of example, with reference to theappended figures, in comparison to a device making it possible toimplement the declogging method according to the prior art.

[0026]FIG. 1 is an overall view in elevation and in vertical section ofa plant for manufacturing uranium oxide by the dry conversion method.

[0027]FIG. 2 is a view in partial vertical section of a filtration unitof the reactor for converting uranium hexafluoride into uraniumoxyfluoride.

[0028]FIG. 3 is a top view, along 3-3 of FIG. 2, of the plate supportingthe filtering cartridges of the filtration unit.

[0029]FIG. 4 is an outline diagram showing the gas routing and flowmeans, allowing gas to enter and exit the upper part of the conversionreactor, for a reactor comprising a declogging device according to theprior art.

[0030]FIG. 5 is a view in vertical section of a filtration unit of auranium hexafluoride conversion reactor comprising a declogging deviceaccording to the invention.

[0031]FIG. 6 is a detailed view of part of FIG. 5 showing the decloggingdevice for implementing the method according to the invention.

[0032]FIGS. 7, 8 and 9 are plane views of three different embodiments ofthe declogging device according to the invention.

[0033]FIG. 1 shows a uranium oxide manufacturing plant generally denotedby the reference 1 and comprising a reactor 2 for converting uraniumhexafluoride into uranium oxyfluoride and a rotary furnace 3 forconverting uranium oxyfluoride into uranium oxide.

[0034] The reactor 2 consists of an enclosure generally placed in avertical arrangement, in which emerges a pipe 5 for injecting reagentgases UF₆ and H₂O and a dilution gas which may be an inert gas such asnitrogen, inside the enclosure of the reactor 2.

[0035] Hydrolysis of uranium hexafluoride UF₆ by steam takes place inthe reactor 2, so as to form powdered uranium oxyfluoride which falls tothe bottom of the reactor 2 and which is taken up by a conveying screw 4which conveys the uranium oxyfluoride in powdered solid form formed inthe reactor 2 to the input of the rotary furnace 3 in which the uraniumoxyfluoride is converted into uranium oxide (mainly UO₂).

[0036] The hydrolysis reaction of uranium hexafluoride produces gaseoushydrofluoric acid HF inside the reactor 2, the gaseous hydrofluoric acidbeing evacuated to the outside of the uranium hexafluoride conversionreactor 2 by two vertical recovery pipes 7, each one connected to afiltration unit 8 of the reactor 2. In this particular embodimentdescribed, each of the filtration units 8 comprises eight filteringcartridges such as 10 comprising a tubular filtering wall arranged withits vertical axis inside the reactor 2, in the outlet part of thereactor.

[0037] As can be seen in particular in FIGS. 2 and 3, the filteringcartridges 10 of each of the filtration units 8 are fastened to ahorizontal plate 9 of the filtration unit which is traversed bythrough-passages of the filtering cartridges 10 and which separate amanifold or head 11 of the filtration unit, in which the recovery pipe 7emerges, from the internal volume of the reactor 2. Each of thefiltering cartridges 10 or filter of the filtration unit 8 is fastenedin a sealed manner to the upper surface of the plate 9, via a fasteningflange. Each of the filtration cartridges 10 is closed at its end placedinside the reactor 2 and open at its opposite end emerging in the head11 of the filtration unit.

[0038] In this way, the gas stream must cross the filtering wall of thefiltration cartridges 10 in order to pass from the enclosure of thereactor 2 to the head 11 communicating with the gas recovery pipe 7.

[0039] As can be seen in FIG. 2, the gas stream 13 which crosses thefiltration wall 12 of the filtering cartridge is separated from theuranium oxyfluoride powder which cannot cross the pores of the wall 12.Some of the uranium oxyfluoride powder is however deposited on the wall12 of the filtering cartridge 10, such that the filtering cartridgegradually clogs up during operation of the reactor.

[0040] In FIG. 4, on the one hand, means for supplying the reactor 2with reagent gas and with dilution gas together with means for removingthe gases formed or present in the reactor 2 and, on the other hand,means making it possible to declog the filtering cartridges 10 of thefiltration units 8 of the reactor 2 are shown schematically for a plantaccording to the prior art.

[0041] The reactor 2 comprises two filtration units 8, each one of theunits itself comprising eight filtration cartridges distributed over thecross section of the filtration unit, as can be seen in FIG. 3 showingthe distribution of the parts for supporting and fastening the filteringcartridges on the plate 9 for fastening filtering cartridges of afiltration unit 8. The plate 9, of substantially square shape, compriseseight through-openings for a filtering cartridge, around each one ofwhich a filtering cartridge rests by means of a flange 14 fastened toits upper end. The eight through-openings of the eight filteringcartridges are arranged on the plate 9 along a central row comprisingtwo cartridges whose axes are placed in a central plane of the plate 9and two outer rows arranged on each side of the central row comprisingthree cartridges, whose axes are placed in two planes parallel to thecentral plane containing the axes of the central row. The plate 9 closesthe head 11 of the filtration unit in its lower part, and the gasrecovery pipe 7 is arranged along the vertical axis of the filtrationunit and thus emerges directly above the central part of the plate 9,between the emerging parts of the two filtering cartridges arranged inthe central row.

[0042] In FIG. 4, the upper part of the reactor 9 for converting uraniumhexafluoride into uranium oxyfluoride together with means for evacuatinggases and means for declogging the filters of the filtration units,according to the prior art, are shown.

[0043] Two filtration units 8 placed in parallel and each comprising apipe 7 for evacuating gas containing hydrofluoric acid to the outside ofthe reactor 2 are arranged in the upper part of the reactor 2. A heatedchamber 15 comprising an insulating wall and a heating resistor 16making it possible to keep the temperature inside the chamber at about150° C. is placed above the reactor 2.

[0044] Two three-way valves 17, each one of which is connected via afirst way 17 a and a pipe 17′ to the evacuation pipe 7 of a filtrationunit 8, emerging in the head 11 of the filtration unit and by a secondway 17 b and a pipe 17″ to a pipe 19 for evacuating gases to theauxiliary filter 18, are arranged inside the chamber 15.

[0045] The third way 17 c of the three-way valves 17 is connected via apipe 21, on which an isolation valve 12′ is placed, to a tank 20containing pressurized nitrogen which can be used to declog the filters10 of the filtration units 8 by the method according to the prior artwhich will be briefly described below.

[0046] The tank 20 contains nitrogen at a pressure of 2.5 bar heated to130° C. by a heating resistor 22.

[0047] During normal operation of the conversion reactor 2, uraniumhexafluoride UF₆, steam and dilution nitrogen are introduced inside thereactor 2 via the pipe 5.

[0048] Uranium oxyfluoride UO₂F₂ and hydrofluoric acid HF, which isremoved by the pipes 7 to the output of the two filtration units 8,which are connected to the three-way valves 17 controlled by the controlcircuit 23, are formed inside the reactor 2. The valves 17 are thencontrolled so that the acid gas passes through the first and the secondways of the three-way valves in order to be delivered to the auxiliaryfilter 18, then recovered.

[0049] The declogging method according to the prior art, which may beimplemented by using the plant shown in FIG. 4, is a method known as“off line”, that is to say a method in which the filtration unit to bedeclogged are placed outside the circuit. To do this, the three-wayvalves 17 of the filtration unit to be declogged are controlled in orderto put the third and the first ways of the three-way valve 17 incommunication. As a result, the gases produced in the reactor 2 can nolonger be evacuated by the filtration unit 8 which is being decloggedand the corresponding evacuation pipe 7.

[0050] Declogging is carried out by opening the isolation valve 21′ ofthe nitrogen feed line, so as to deliver a jet of pressurized decloggingnitrogen at a temperature of 130° C. into the connection pipe 171 andinto the evacuation pipe 7 of the filtration unit 8 which is beingdeclogged.

[0051] The declogging nitrogen is injected into the head 11 of thefiltration unit, directly above the filtering cartridges 10 located inthe central part of the plate 9 of the filtration unit. As a result,both central filtering cartridges of the filtration unit may besufficiently declogged, while the six furthermost cartridges are onlypartially declogged.

[0052] Furthermore, in order to declog a filtration unit, it isnecessary to inject a volume of declogging nitrogen into the filtrationunit which is relatively high and generally about 50 liters (atatmospheric pressure), during the declogging which is carried out byopening the isolation valve of the nitrogen tank for a duration of about0.5 seconds.

[0053] As a result, the second filtration unit still in operation duringthe declogging must evacuate not only all the gases produced in thereactor 2 but also the additional stream of declogging nitrogen injectedinto the first unit, which causes a general overpressure in the reactor.

[0054] The known method of the prior art therefore has reducedefficiency with regard to declogging the filters away from the centralpart of the filtration unit.

[0055] Furthermore, carrying out “off line” declogging on a filtrationunit requires abnormal operation of the second filtration unit and ofthe corresponding circuit for evacuating the gases containing thehydrofluoric acid.

[0056] According to the invention, a new declogging method is provided,which makes it possible to keep the filtration unit which is beingdeclogged in operation during the declogging.

[0057] Such a method is called an “on line” method.

[0058]FIGS. 5 and 6 show a filtration unit of a reactor for convertinguranium hexafluoride into uranium oxyfluoride equipped with a decloggingdevice making it possible to implement the method according to theinvention.

[0059] The corresponding elements of the plant shown in FIGS. 5 and 6,on the one hand, and in FIGS. 1 to 4, on the other hand, are denoted bythe same references.

[0060] The filtration units 8 of the plant shown in FIGS. 5 and 6 areproduced in the same way as the filtration units described with respectto FIGS. 2 and 3. These filtration units each comprise eight filters orfiltering cartridges 10 supported by a plate 9 traversed bythrough-openings for filtering cartridges, the plate 9 separating thehead or manifold 11 of the filtration unit in which the gas recoverypipe emerges from the internal space of the reactor 2 in which thefiltering cartridges 10 are introduced.

[0061] During normal operation of the conversion reactor 2, gaseoushydrofluoric acid and other gases (steam, hydrogen, etc.) cross theporous wall 12 of the filtering cartridges 10, in the internal space ofthe reactor, as shown by the arrow 13, and particles 13′ of uraniumoxyfluoride in powder form fall back into the bottom of the reactor 2.However, progressive clogging of the wall 12 of the filtering cartridgesoccurs, which requires periodic declogging of the cartridges.

[0062] According to the invention, declogging is carried out by one ormore very brief successive jets of declogging nitrogen delivered alongthe axis of each of the filtering cartridges 10, at a sonic velocity.Declogging nitrogen is injected while keeping the filtration unit whichis being declogged in operation, that is to say while maintaining thepassage of mixed hydrofluoric acid and gases through the filtration unitfor their evacuation through the pipe 7 connected to the head 11 of thefiltration unit.

[0063] The head 11 of a filtration unit 8 comprises a head wall 25 whichis fastened, in a parallel arrangement, above the plate 9 separating thehead 11 from the internal volume of the reactor 2, via a flange 25′resting on the plate 9. Screw and bolt assemblies 26 make it possible tofasten and clamp, one against the other, the flange 25′ of the wall 25of the head, a peripheral part of the plate 9 supporting the filters 10and a flange 2′ secured to the upper part of the wall of the reactor 2.Seals are inserted between the flanges clamped one against the other oneach side of the plate 9. The wall 25 of the plate 11 carries a set ofeight nozzles 28 for injecting declogging nitrogen, each one along theaxis 10′ of a filtering cartridge 10. Each of the nozzles 28 may beconnected by a pipe 27 to one solenoid valve of a set of solenoid valves24 for distributing declogging nitrogen, valves fastened to a supportabove the filtration units 8.

[0064] Each of the nozzles 28 for injecting declogging nitrogen isassociated with a filtering cartridge 10 and placed so that the axis ofthe nozzle 28 is directed along the axis 10′ of the filtering cartridge,the injection end of the nozzle 28 being slightly above the uppersurface of a flange 29 for supporting and fastening the filteringcartridge 10 to the upper surface of the plate 9 of the head 11.

[0065] The pipe of the nozzle 28 may be cylindrical or ofconvergent-divergent shape; the nozzle may be placed in the head 11,immediately upstream and in a coaxial position with respect to anopening 30 traversing the support flange 29 of the filtering cartridge10 forming a venturi, that is to say a convergent-divergent pipeproviding additional acceleration of a jet of declogging nitrogendelivered under pressure into the nozzle 28 for a very short duration.Thus, declogging is carried out by a pulsed jet at sonic velocity, thatis to say at a velocity substantially equal to or slightly greater thanthe speed of sound.

[0066] In a variant embodiment, the nozzle may be inserted inside thefiltering cartridge and placed in a central region half way up thefiltering cartridge. Thus penetration of the cleaning gas jet into thefilter cartridge is ensured.

[0067] We will now describe various methods of feeding the decloggingnozzles 28 of the filtration units with respect to FIGS. 7, 8 and 9.

[0068] The different variants of the system for feeding the decloggingnozzles make it possible to feed the declogging nozzles in groups orindividually. The declogging nozzles, which are individual nozzles foreach of the filtering cartridges, may therefore be fed simultaneously orsuccessively depending on the feed system chosen.

[0069] Typical conditions for producing and operating a means fordeclogging a filtering cartridge consisting of a nozzle 28 and a venturi30 will be given below.

[0070] For a nozzle having a convergent-divergent pipe, the diameter ofthe neck of the nozzle may be about 10 mm. The angle of opening of thedivergent part may be between 5 and 150 and, for example, about 100.

[0071] The declogging frequency is adjusted according to the increase inpressure drop of the reactor and the duration of opening of thedeclogging valve 24 connected to the nozzle 28 by the pipe 27 may be of0.1 seconds to 0.5 seconds, for example 0.14 seconds or even 0.2seconds.

[0072] The excess pressure of the declogging nitrogen in the nitrogenstorage tank is from 2 to 10 bar, and preferably about 6 bar.

[0073] For a nitrogen pressure of 6 bar and a nozzle having an orificeof about 10 mm, the volume of nitrogen injected into each of thedecloggers is 10 liters, for a duration of 0.14 seconds, and 15 litersfor a duration of 0.2 seconds. It should be noted that the volumes ofdeclogging nitrogen injected during each of the declogging operations bypulsed jet are substantially less than the volume injected during adeclogging operation according to the prior art.

[0074] The nozzle is produced such that the velocity of the gas jetinjected at the output of the venturi 30 is greater than 330 m/s, whichlocates the jet in the region of sonic velocities.

[0075] Depending on the type of filter used and the operating mode ofthe conversion plant, the characteristic parameters of the pulsed jet,that is its velocity and its duration, are able to be different from thevalues given above.

[0076] However, in all cases, the velocity of the declogging jet at theinput of the filtration cartridge will be greater than 300 m/s and theduration of injection of the jet at sonic velocity will be less than 1second.

[0077] As has been indicated above, the nozzles and therefore thedeclogging pipe can be fed in different ways which are described withrespect to FIGS. 7, 8 and 9.

[0078] In FIGS. 7, 8 and 9, the corresponding elements bear the samereferences and the corresponding elements shown in FIGS. 7, 8 and 9 onthe one hand and in FIG. 4 on the other hand are also denoted by thesame references.

[0079] In all cases, within the scope of the method according to theinvention, a plant is used which comprises means for evacuatinghydrofluoric acid mixed with steam, hydrogen and dilution nitrogen andmeans for distributing declogging nitrogen which are simpler than thosewhich have been described for the prior art, with respect to FIG. 4.

[0080] In FIG. 7, a set of means for distributing declogging nitrogeninto the nozzles 28 of a declogging plant according to the invention, asdescribed above with regard to FIGS. 5 and 6, is shown.

[0081] The means for distributing nitrogen shown in FIG. 7 are made sothat the nitrogen can be delivered successively into each of the nozzles28 for the successive declogging of each of the filtering cartridges 10of each of the filtering units of the plant.

[0082] Two variants of the nitrogen distribution means making itpossible to feed all the declogging nozzles 28 of the filters of afiltration unit or the declogging nozzles of only some of the filters ofa filtration unit (for example half of the filters) simultaneously willbe described with respect to FIGS. 8 and 9, respectively.

[0083] As can be seen in FIG. 7, the means for distributing nitrogeninto the nozzles 28 comprise a nitrogen tank 31 consisting of alarge-diameter insulated pipe.

[0084] The nitrogen tank 31 with a 20 to 30 liter capacity containsnitrogen at a pressure which may be between 2 and 10 bar and which is,for example, 6 bar and a temperature of about 130° C. Solenoid valves24, distributing nitrogen into each of the pipes 27 feeding a nozzle 28via a hose 32, are placed on one side of the pipe forming the tank 31.

[0085] For the embodiment shown in FIG. 7, sixteen solenoid valves 24,each one associated with one of the sixteen nozzles for declogging thefilters of the two filtration units, are shown. Each of the solenoidvalves 24 is connected to the nitrogen tank 31 via a pipe 33 on which amanual stop valve 34 is arranged, optionally enabling one or more valvesof the nitrogen tank to be isolated.

[0086] The solenoid valves 24 may be automatically controlled insequence, in order to feed declogging nitrogen to each of the nozzles 28in succession, for a predetermined duration of, for example, between 0.1and 0.5 seconds.

[0087] For the embodiment of the distribution means shown in FIG. 8, asingle solenoid valve 36 (or 36′), connected to the nitrogen tank 31 bya pipe 37, on which a manual stop valve 37 a is arranged, feeds amanifold 35 (or 35′) with nitrogen, to which manifold the pipes 27feeding the eight declogging nozzles 28 of a filtration unit areconnected by hoses 38. The declogging nozzles 28 of the filters of thetwo filtration units are fed with nitrogen from two solenoid valves 36and 36′, each one associated with a filtration unit, which valve maysuccessively be controlled for their opening and the provision ofnitrogen to each of the manifolds 35 and 35′, in succession.

[0088] In FIG. 9, nitrogen distribution means are shown according to asecond embodiment which comprises, for each of the two filtration units,two solenoid valves 36 a and 36 b (or 36′a and 36′b), each one connectedto a manifold 35 a and 35 b (or 35′a and 35′b) to which the pipes 27 offour nozzles 28 of four filters of a filtration unit are connected viahoses. The distribution assembly therefore comprises four solenoidvalves which can be automatically controlled in succession, in order tosuccessively declog the filters 10, for each set of four filters of afiltration unit.

[0089] More generally, it is possible to design nitrogen distributionplants making it possible successively to declog sets of n filters outof the N filters of a filtration unit of a reactor for producing uraniumoxyfluoride.

[0090] It should be noted that, even when simultaneously declogging theeight cartridges 10 of a filtration unit or four cartridges or moregenerally n filtering cartridges, declogging is carried out individuallyin each of the filtering cartridges, unlike the declogging according tothe prior art.

[0091] Furthermore, each of the filtering cartridges is individuallydeclogged without stopping the evacuation of the hydrofluoric acid bythe filtration unit. In particular, it is not necessary to use a meansfor routing evacuated gases such as a three-way valve. While thedeclogging nitrogen is being injected for a very short duration, thisinjection provides a very brief temporary stop of the evacuation of thegases in the filtering cartridges, the injected declogging nitrogencreating a shockwave.

[0092] The evacuation of gases starts back up soon after injection ofthe declogging gases, through the venturis 30 forming the gas outletsfrom the filtering cartridges 10.

[0093] In all cases, the method according to the invention makes itpossible to obtain efficient and uniform declogging of each of thefilters of the filtration unit by means of an “on line” method, that isto say without interrupting the evacuation of the hydrofluoric acidmixed with the steam and dilution nitrogen and hydrogen to the outsideof the reactor.

[0094] For individual successive declogging, or declogging in groups ofeight, four or n filters of a filtration unit comprising N filters,where n<N, evacuation of the gases is carried out completelycontinuously, through the filtering cartridges in which declogging isnot carried out, during the very short duration of the pulseddeclogging.

[0095] The invention is not limited to the embodiments which have beendescribed.

[0096] It is possible to use means for injecting inert declogging gasother than those which have been described, in order to obtain a jet ofinert declogging gas at sonic or slightly supersonic velocity producinga shockwave inside the filter providing very good declogging bydetaching the particles retained on the wall 12 of the filters.

[0097] Instead of nitrogen, any other inert gas such as argon can beused, in order to carry out declogging by pulsed jet at sonic velocity.

[0098] It is possible to use means for distributing and routing thedeclogging gas other than the means described.

[0099] The invention is applicable to any uranium oxide production plantcomprising a reactor for converting uranium hexafluoride intooxyfluoride having an outlet part on which filtration units arearranged, each one comprising one or more filters in parallel.

[0100] The conversion reactor may comprise any number of filtrationunits themselves comprising any number of filters.

[0101] For a plant comprising at least one filtration unit comprising aplurality of filters arranged in parallel, the means of injecting inertdeclogging gas comprise an injection nozzle placed in axial alignmentwith each of the filters of the plurality of filters.

[0102] Instead of solenoid valves, it is possible to use any other typeof automatic control valve for distributing the declogging gas in thedeclogging nozzles, whether individually or in groups.

[0103] For existing industrial plants, it is possible to substantiallyincrease the hourly output of the uranium hexafluoride conversion plant,by using a declogging method according to the invention.

[0104] For plants according to the prior art, the hexafluoride outputhad to be limited to a hexafluoride output of less than 100 kg/h andgenerally close to 75 kg/h. For plants having a declogging deviceaccording to the invention, twice the output can be treated, that is tosay 150 kg/h.

1. A method of declogging at least one filter of a plant for producinguranium oxide from uranium hexafluoride, comprising a reactor (2) inwhich gaseous hexafluoride UF₆ and steam are injected, uraniumoxyfluoride UO₂F₂ in powder form and gaseous hydrofluoric acid HF areformed, uranium oxyfluoride UO₂F₂ conveyed by gases containing gaseoushydrofluoric acid HF is separated, in an outlet part of the reactor (2),in at least one filtration unit (8) comprising at least one filter (10)having a tubular filtration wall (12) arranged with its axis (10′)vertical, and a gas containing hydrofluoric acid is evacuated to theoutside of the reactor (2), the declogging of the filter consisting inseparating, from the wall (12) of the filter, uranium oxyfluoride UO₂F₂particles deposited on the wall (12) by a stream of inert gas such asnitrogen, injected into the filter (10) counter-currentwise to the flowof evacuated gas containing the hydrofluoric acid HF, characterized inthat inert gas is injected along the axis (10′) of the wall of thefilter (10) in the form of a jet with a velocity greater than 300 m/sfor a duration of less than 1 second.
 2. The method as claimed in claim1, characterized in that the inert declogging gas jet has a velocitygreater than 330 m/s.
 3. The method as claimed in either of claims 1 and2, characterized in that the duration of injecting inert declogging gasinto at least one filter (10) is between 0.1 and 0.5 seconds, the inertgas being at a temperature close to 130° C. and at an initial pressureof between 2 and 10 bar and, for example, of about 6 bar.
 4. The methodas claimed in any one of claims 1 to 3, characterized in that individualinjections of inert declogging gas are carried out, along their axis(10′), inside each of a plurality of filters (10) of a filtration unit(8).
 5. The method according to claim 4, for a filtration plantcomprising at least one filtration unit (8) comprising N filters (10)arranged in parallel, characterized in that declogging is carried out insuccessive groups of n filters (where n<N) of each of the filtrationunits (8).
 6. The declogging method according to claim 4, characterizedin that declogging is carried out by injecting an inert gas successivelyinto each of the filters of each of at least one filtration unit (8) ofthe reactor (2).
 7. A device for declogging at least one filter (10) ofa plant (1) for producing uranium oxide from uranium hexafluoride,comprising a reactor (2) including a jacket having a generally verticalarrangement, a pipe (5) for injecting uranium hexafluoride, steam and adilution gas entering the jacket of the reactor (2) together with atleast one unit (8) for filtering gases evacuated from the reactor (2)comprising a manifold (11) connected to a pipe (7) for evacuating gasesto outside the reactor (2) in which emerges the outlet end of at leastone filter (10) having a tubular wall (12) with a vertical axis (10′)supported by a plate (9) separating the gas manifold (11) from the innerspace of the jacket of the reactor (2) in which the filter (10) isplaced, the declogging device comprising a reservoir (20) for storingpressurized inert declogging gas and means for distributing and routinginert declogging gas toward the at least one filter (10) of the at leastone filtration unit (8) of the reactor (2), characterized in that itcomprises, aligned with the vertical axis (10′) of the wall (12) of thefilter (10), a nozzle (28) for injecting inert gas, the nozzle connectedby a pipe (27) for feeding inert declogging gas to at least onedistribution element (24, 36, 36′, 36 a, 36 b, 36′a, 36′b).
 8. Thedevice as claimed in claim 7, characterized in that the nozzle (28) forinjecting declogging gas is arranged in the manifold (11) of thefiltration unit (8), opposite an outlet end of the filter (10) and thata flange (29) for supporting and fastening the filter (10) resting onthe plate (9) of the manifold (11) has a through-opening (30) coaxialwith the nozzle (28) and constituting a venturi for accelerating thedeclogging gas injected into the filter (11) along the axial direction(10′) via the nozzle (28).
 9. The device as claimed in claim 7,characterized in that the nozzle (28) is arranged inside and close tothe central part of the tubular part (12) of the filter (10).
 10. Thedevice as claimed in any one of claims 7 to 9, characterized in that thenozzle (28) has the shape of a convergent-divergent pipe.
 11. The deviceas claimed in either of claims 7 and 10, for a plant comprising at leastone filtration unit (8) having a plurality of filters (10) arranged inparallel, characterized in that the declogging gas injection meanscomprising an injection nozzle (28) placed in axial alignment (10′) withthe jacket (12) of a filter (10) are associated with each of the filters(10) of the at least one filtration unit (8).
 12. The device as claimedin claim 11, characterized in that each of the nozzles (28) of a meansfor declogging a filter (10) is connected via a feed pipe (27, 32) to anindividual means of distributing inert declogging gas consisting of anindividual automatic control valve (24).
 13. The device as claimed inclaim 11, characterized in that each of the nozzles (28) of each of themeans of injecting declogging gas into a filter (10) is connected via apipe (27) to distribution means (36, 36′, 36 a, 36 b, 36′a, 36′b)ensuring the simultaneous feeding of n nozzles (28) out of N nozzles ofa filtration unit (8).